The VarA-CsrA regulatory pathway influences cell shape in Vibrio cholerae

Despite extensive studies on the curve-shaped bacterium Vibrio cholerae, the causative agent of the diarrheal disease cholera, its virulence-associated regulatory two-component signal transduction system VarS/VarA is not well understood. This pathway, which mainly signals through the downstream protein CsrA, is highly conserved among gamma-proteobacteria, indicating there is likely a broader function of this system beyond virulence regulation. In this study, we investigated the VarA-CsrA signaling pathway and discovered a previously unrecognized link to the shape of the bacterium. We observed that varA-deficient V. cholerae cells showed an abnormal spherical morphology during late-stage growth. Through peptidoglycan (PG) composition analyses, we discovered that these mutant bacteria contained an increased content of disaccharide dipeptides and reduced peptide crosslinks, consistent with the atypical cellular shape. The spherical shape correlated with the CsrA-dependent overproduction of aspartate ammonia lyase (AspA) in varA mutant cells, which likely depleted the cellular aspartate pool; therefore, the synthesis of the PG precursor amino acid meso-diaminopimelic acid was impaired. Importantly, this phenotype, and the overall cell rounding, could be prevented by means of cell wall recycling. Collectively, our data provide new insights into how V. cholerae use the VarA-CsrA signaling system to adjust its morphology upon unidentified external cues in its environment.     

The AgI/II Family Adhesin AspA Is Required for Respiratory Infection by Streptococcus pyogenes

Streptococcus pyogenes (GAS) is a human pathogen that causes pharyngitis and invasive diseases such as toxic shock syndrome and sepsis. The upper respiratory tract is the primary reservoir from which GAS can infect new hosts and cause disease. The factors involved in colonisation are incompletely known however. Previous evidence in oral streptococci has shown that the AgI/II family proteins are involved. We hypothesized that the AspA member of this family might be involved in GAS colonization. We describe a novel mouse model of GAS colonization of the nasopharynx and lower respiratory tract to elucidate these interactions. We used two clinical M serotypes expressing AspA, and their aspA gene deletant isogenic mutants in experiments using adherence assays to respiratory epithelium, macrophage phagocytosis and neutrophil killing assays and in vivo models of respiratory tract colonisation and infection. We demonstrated the requirement for AspA in colonization of the respiratory tract. AspA mutants were cleared from the respiratory tract and were deficient in adherence to epithelial cells, and susceptible to phagocytosis. Expression of AspA in the surrogate host Lactococcus lactis protected bacteria from phagocytosis. Our results suggest that AspA has an essential role in respiratory infection, and may function as a novel anti-phagocytic factor.     

Bacillus subtilis CwlQ (previous YjbJ) is a bifunctional enzyme exhibiting muramidase and soluble-lytic transglycosylase activities

CwlQ (previous YjbJ) is one of the putative cell wall hydrolases in Bacillus subtilis. Its domain has an amino acid sequence similar to the soluble-lytic transglycosylase (SLT) of Escherichia coli Slt70 and also goose lysozyme (muramidase). To characterize the enzyme, the domain of CwlQ was cloned and expressed in E. coli. The purified CwlQ protein exhibited cell wall hydrolytic activity. Surprisingly, RP-HPLC, mass spectrometry (MS), and MS/MS analyses showed that CwlQ produces two products, 1,6-anhydro-N-acetylmuramic acid and N-acetylmuramic acid, thus indicating that CwlQ is a bifunctional enzyme. The site-directed mutagenesis revealed that glutamic acid 85 (Glu-85) is an amino acid residue essential to both activities.     

Escherichia coli phosphoenolpyruvate carboxylase: studies on the mechanism of multiple allosteric interactions.

Some kinetic studies of the interactions between Escherichia coli phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxylase (phosphorylating) EC 4.1.1.31) acetyl coenzyme A, fructose 1,6-bisphosphate, and aspartate were performed. Activation of the enzyme by fructose 1,6-bisphosphate is anomalous by comparison with acetyl coenzyme A in that it confers hysteretic properties on the enzyme. In the presence of both activators and aspartate, hysteresis is observed also, but the approach to optimum catalytic activity can be fit to an equation for a second-order reaction with respect to enzyme concentration. Since, however, hysteresis is not a result of any apparent association-dissociation reaction, the apparent fit to a second-order kinetic equation is probably not real but is the result of a multistep activation mechanism. Hysteresis is not eliminated by preincubation of the enzyme with fructose 1,6-bisphosphate, acetyl coenzyme A, or phosphoenolpyruvate singly or in any pair of combinations. Hysteresis is associated, therefore, with the slow conformation change from the inactive species to the active species under the influence of all three of those reactants. The enzyme complex resulting from the binding of each activator, including phosphoenolpyruvate, has an increased affinity for the other activators. A kinetic method for estimating the relative changes in affinity of these complexes for some of the other reactants is presented. At concentrations of the activators below their K_a, synergistic effects are evident, particularly in their ability to relieve aspartate inhibition. Aspartate inhibition is competitive with acetyl coenzyme A both in the absence and in the presence of low concentrations of fructose 1,6-bisphosphate. Increasing the concentrations of fructose 1,6-bisphosphate results in an increase in the apparent K_l for aspartate, suggesting that synergistic activation by fructose 1,6-bisphosphate is a result of the increased affinity of the fructose 1,6-bisphosphate-enzyme complex for acetyl coenzyme A, and a shift in the concentration of enzyme species away from the one(s) to which aspartate can bind most easily. In the presence of fructose 1,6-bisphosphate alone optimal activation can be achieved, but the concentrations required in vitro are high and suggest that fructose 1,6-bisphosphate alone does not function in that capacity physiologically, but primes the enzyme for more effective activation by acetyl coenzyme A and/or phosphoenolpyruvate.     

Amplification of the rudimentary gene in a PALA-resistant Drosophila cell line

In Drosophila melanogaster the rudimentary locus encodes for a multifunctional protein catalyzing the first three enzymatic activities of pyrimidine biosynthesis. Cell lines were selected which were resistant to PALA (N-(phosphonoacetyl)-L-aspartate), a specific inhibitor of aspartate transcarbamylase, the second enzyme of this pathway. In a cell line where the enzyme production is increased 5 times, Southern blot analyses show that the rudimentary gene and surrounding regions are amplified about 5 times. In this case gene amplification could therefore account for the observed enzyme overproduction.     

The antibacterial activity of phospholipase A2 type IIA is regulated by the cooperative lipid chain melting behavior in Staphylococcus aureus

As one of its primary physiological functions, sPLA₂-IIA appears to act as an antibacterial agent. In particular, sPLA₂-IIA shows high activity towards Gram-positive bacteria such as Staphylococcus aureus (S. aureus). This antibacterial activity results from the preference of the enzyme towards membranes enriched in anionic lipids, which is a common feature of bacterial membranes. An intriguing aspect observed in a variety of bacterial membranes is the presence of a broad but cooperative lipid chain melting event where the lipids in the membrane transition from a solid-ordered (so) into a liquid-disordered (ld) state close to physiological temperatures. It is known that the enzyme is sensitive to the level of lipid packing, which changes sharply between the so and the ld states. Therefore, it would be expected that the enzyme activity is regulated by the bacterial membrane thermotropic behavior. We determine by FTIR the thermotropic lipid chain melting behavior of S. aureus and find that the activity of sPLA₂-IIA drops sharply in the so state. The activity of the enzyme is also evaluated in terms of its effects on cell viability, showing that cell survival increases when the bacterial membrane is in the so state during enzyme exposure. These results point to a mechanism by which bacteria can develop increased resistance towards antibacterial agents that act on the membrane through a cooperative increase in the order of the lipid chains. These results show that the physical behavior of the bacterial membrane can play an important role in regulating physiological function in an in vivo system.     

Amino acid-dependent growth of Campylobacter jejuni: key roles for aspartase (AspA) under microaerobic and oxygen-limited conditions and identification of AspB (Cj0762), essential for growth on glutamate

Amino acids are key carbon and energy sources for the asaccharolytic food-borne human pathogen Campylobacter jejuni . During microaerobic growth in amino acid rich complex media, aspartate, glutamate, proline and serine are the only amino acids significantly utilized by strain NCTC 11168. The catabolism of aspartate and glutamate was investigated. An aspartase ( aspA ) mutant (unable to utilize any amino acid except serine) and a Cj0762 c ( aspB ) mutant lacking aspartate:glutamate aminotransferase (unable to utilize glutamate), were severely growth impaired in complex media, and an aspA sdaA mutant (also lacking serine dehydratase) failed to grow in complex media unless supplemented with pyruvate and fumarate. Aspartase was shown by activity and proteomic analyses to be upregulated by oxygen limitation, and aspartate enhanced oxygen-limited growth of C. jejuni in an aspA -dependent manner. Stoichiometric aspartate uptake and succinate excretion involving the redundant DcuA and DcuB transporters indicated that in addition to a catabolic role, AspA can provide fumarate for respiration. Significantly, an aspA mutant of C. jejuni 81-176 was impaired in its ability to persist in the intestines of outbred chickens relative to the parent strain. Together, our data highlight the dual function of aspartase in C. jejuni and suggest a role during growth in the avian gut.     

Physcomitrella patens has lipoxygenases for both eicosanoid and octadecanoid pathways

Mosses have substantial amounts of long chain C20 polyunsaturated fatty acids, such as arachidonic and eicosapentaenoic acid, in addition to the shorter chain C18 α-linolenic and linoleic acids, which are typical substrates of lipoxygenases in flowering plants. To identify the fatty acid substrates used by moss lipoxygenases, eight lipoxygenase genes from Physcomitrella patens were heterologously expressed in Escherichia coli, and then analyzed for lipoxygenase activity using linoleic, α-linolenic and arachidonic acids as substrates. Among the eight moss lipoxygenases, only seven were found to be enzymatically active in vitro, two of which selectively used arachidonic acid as the substrate, while the other five preferred α-linolenic acid. Based on enzyme assays using a Clark-type oxygen electrode, all of the active lipoxygenases had an optimum pH at 7.0, except for one with highest activity at pH 5.0. HPLC analyses indicated that the two arachidonic acid lipoxygenases form (12S)-hydroperoxy eicosatetraenoic acid as the main product, while the other five lipoxygenases produce mainly (13S)-hydroperoxy octadecatrienoic acid from α-linolenic acid. These results suggest that mosses may have both C20 and C18 based oxylipin pathways.     

The effect of sulfhydryl group reagents on the permeation of mitochondrial aspartate aminotransferase into mitochondria in vitro.

When mitochondria are incubated with radioactively labeled mitochondrial aspartate aminotransferase (EC 2.6.1.1), the enzyme is taken up into the organelles. Mersalyl and p-hydroxymercuriphenyl sulfonic acid, but not N-ethylmaleimide or ethacrynic acid, decrease the extent of this uptake. Inhibition of the uptake by low concentrations of mercurial reagents is due to blockage of a single sulfhydryl group per monomer of the enzyme. Blockage of mitochondrial thiols does not inhibit uptake of the enzyme. A single sulfhydryl group out of a total of six per monomer of the native enzyme reacts with 5,5′-dithiobis-(2-nitrobenzoic acid). This is the same sulfhydryl group that reacts with low levels of mercurial reagents with consequent inhibition of uptake of the enzyme into mitochondria but without effect on the catalytic activity. N-Ethylmaleimide does not react with this group. N-Ethylmaleimide reacts with a different sulfhydryl group with concomitant decrease in enzymic activity but with no effect on uptake of the enzyme into mitochondria. High levels of mercurial reagents similarly decrease enzymic activity. Unlike the effect on uptake into mitochondria, the inhibition by mercurial reagents of enzymic activity is not reversed by treatment with cysteine. The significance of these observations with respect to the mechanism of uptake of aspartate aminotransferase into mitochondria is discussed, and comparisons are made between the reactivities of sulfhydryl groups in rat liver aspartate aminotransferase and in the enzymes from other animals.     

Molecular cloning and enzymological characterization of pyridoxal 5′-phosphate independent aspartate racemase from hyperthermophilic archaeon <Emphasis Type="Italic">Thermococcus</Emphasis><Emphasis Type="Italic">litoralis</Emphasis> DSM 5473

We succeeded in expressing the aspartate racemase homolog gene from Thermococcus litoralis DSM 5473 in Escherichia coli Rosetta (DE3) and found that the gene encodes aspartate racemase. The aspartate racemase gene consisted of 687 bp and encoded 228 amino acid residues. The purified enzyme showed aspartate racemase activity with a specific activity of 1590 U/mg. The enzyme was a homodimer with a molecular mass of 56 kDa and did not require pyridoxal 5′-phosphate as a coenzyme. The enzyme showed aspartate racemase activity even at 95 °C, and the activation energy of the enzyme was calculated to be 51.8 kJ/mol. The enzyme was highly thermostable, and approximately 50 % of its initial activity remained even after incubation at 90 °C for 11 h. The enzyme showed a maximum activity at a pH of 7.5 and was stable between pH 6.0 and 7.0. The enzyme acted on l-cysteic acid and l-cysteine sulfinic acid in addition to d- and l-aspartic acids, and was strongly inhibited by iodoacetic acid. The site-directed mutagenesis of the enzyme showed that the essential cysteine residues were conserved as Cys83 and Cys194. d-Forms of aspartic acid, serine, alanine, and valine were contained in T. litoralis DSM 5473 cells.     

The role of Cys108 in Trigonopsis variabilisd-amino acid oxidase examined through chemical oxidation studies and point mutations C108S and C108D

Oxidative modification of Trigonopsis variabilisd-amino acid oxidase in vivo is traceable as the conversion of Cys108 into a stable cysteine sulfinic acid, causing substantial loss of activity and thermostability of the enzyme. To simulate native and modified oxidase each as a microheterogeneity-resistant entity, we replaced Cys108 individually by a serine (C108S) and an aspartate (C108D), and characterized the purified variants with regard to their biochemical and kinetic properties, thermostability, and reactivity towards oxidation by hypochlorite. Tandem MS analysis of tryptic peptides derived from a hypochlorite-treated inactive preparation of recombinant wild-type oxidase showed that Cys108 was converted into cysteine sulfonic acid, mimicking the oxidative modification of native enzyme as isolated. Colorimetric titration of protein thiol groups revealed that in the presence of ammonium benzoate (0.12 mM), the two muteins were not oxidized at cysteines whereas in the wild-type enzyme, one thiol group was derivatized. Each site-directed replacement caused a conformational change in d-amino acid oxidase, detected with an assortment of probes, and resulted in a turnover number for the O₂-dependent reaction with D-Met which in comparison with the corresponding wild-type value was decreased two- and threefold for C108S and C108D, respectively. Kinetic analysis of thermal denaturation at 50 °C was used to measure the relative contributions of partial unfolding and cofactor dissociation to the overall inactivation rate in each of the three enzymes. Unlike wild-type, C108S and C108D released the cofactor in a quasi-irreversible manner and were therefore not stabilized by external FAD against loss of activity. The results support a role of the anionic side chain of Cys108 in the fine-tuning of activity and stability of d-amino acid oxidase, explaining why C108S was a surprisingly poor mimic of the native enzyme.     

Identification of the YfgF MASE1 domain as a modulator of bacterial responses to aspartate

Complex 3′-5′-cyclic diguanylic acid (c-di-GMP) responsive regulatory networks that are modulated by the action of multiple diguanylate cyclases (DGC; GGDEF domain proteins) and phosphodiesterases (PDE; EAL domain proteins) have evolved in many bacteria. YfgF proteins possess a membrane-anchoring domain (MASE1), a catalytically inactive GGDEF domain and a catalytically active EAL domain. Here, sustained expression of the Salmonella enterica spp. Enterica ser. Enteritidis YfgF protein is shown to mediate inhibition of the formation of the aspartate chemotactic ring on motility agar under aerobic conditions. This phenomenon was c-di-GMP-independent because it occurred in a Salmonella strain that lacked the ability to synthesize c-di-GMP and also when PDE activity was abolished by site-directed mutagenesis of the EAL domain. YfgF-mediated inhibition of aspartate chemotactic ring formation was impaired in the altered redox environment generated by exogenous p-benzoquinone. This ability of YfgF to inhibit the response to aspartate required a motif, ²¹³Lys-Lys-Glu²¹⁵, in the predicted cytoplasmic loop between trans-membrane regions 5 and 6 of the MASE1 domain. Thus, for the first time the function of a MASE1 domain as a redox-responsive regulator of bacterial responses to aspartate has been shown.     

Specific inhibition of the aspartate aminotransferase of Plasmodium falciparum

Aspartate aminotransferases (AspATs; EC 2.6.1.1) catalyze the conversion of aspartate and α-ketoglutarate into oxaloacetate and glutamate and are key enzymes in the nitrogen metabolism of all organisms. Recent findings suggest that the plasmodial enzyme [Plasmodium falciparum aspartate aminotransferase (PfAspAT)] may also play a pivotal role in energy metabolism and in the de novo biosynthesis of pyrimidines. However, while PfAspAT is a potential drug target, the high homology between the active sites of currently available AspAT structures hinders the development of specific inhibitors of these enzymes. In this article, we report the X-ray structure of the PfAspAT homodimer at a resolution of 2.8 Å. While the overall fold is similar to the currently available structures of other AspATs, the structure presented shows a significant divergence in the conformation of the N-terminal residues. Deletion of these divergent PfAspAT N-terminal residues results in a loss of activity for the recombinant protein, and addition of a peptide containing these 13 N-terminal residues results in inhibition both in vitro and in a lysate isolated from cultured parasites, while the activity of human cytosolic AspAT is unaffected. The finding that the divergent N-terminal amino acids of PfAspAT play a role in catalytic activity indicates that specific inhibition of the enzyme may provide a lead for the development of novel compounds in the treatment of malaria. We also report on the localization of PfAspAT to the parasite cytosol and discuss the implications of the role of PfAspAT in the supply of malate to the parasite mitochondria.     

Role of berberine in ameliorating Schistosoma mansoni-induced hepatic injury in mice

Background

Schistosomiasis is caused by helminth parasites of the genus Schistosoma. Berberine chloride (BER), an isoquinoline alkaloid, has been used in vivo for its antiparasitic, antioxidant and hepatoprotective properties. In this study, the protective effect of BER and praziquantel has been compared for the extent of schistosomiasis-induced oxidative stress in hepatic tissue of mice.

Results

S. mansoni was able to induce inflammation and injury to the liver, evidenced (i) by an increase in inflammatory cellular infiltrations, dilated sinusoids and vacuolated hepatocytes, (ii) by decreased levels of alanine and aspartate aminotransferases and increased levels of alkaline phosphatase, γ-glutamyl transferase in the liver homogenate, (iii) by increased production of nitric oxide and thiobarbituric acid reactive substances, and (iv) by lowered glutathione levels and decreased activities of catalase and superoxide dismutase, respectively. All these infection-induced parameters were significantly altered during BER treatment. In particular, berberine counteracted the S. mansoni-induced loss of glutathione and the activities of catalase and superoxide dismutase.

Conclusion

Based on these results, it is concluded that berberine could ameliorate pre-existing liver damage and oxidative stress conditions due to schistosomiasis.     

On the detection of homotropic effects in enzymes of low co-operativity. Application to modified aspartate transcarbamoylase

In contrast with the ease of observing heterotropic effects in allosteric enzymes of low co-operativity, the detection of homotropic effects is often difficult. As a consequence, erroneous conclusions about the uncoupling of homotropic and heterotropic effects can result unless sensitive techniques are used for analyzing the kinetic data. Simulations of experiments as well as actual measurements on the allosteric enzyme, aspartate transcarbamoylase, of Escherichia coli and some of its modified forms, were performed in attempts to develop stringent diagnostic procedures for the detection of homotropic effects in enzymes of low co-operativity. The analyses show that direct saturation plots (velocity versus substrate concentration), double reciprocal plots, and Hill plots yield misleading results in that the co-operativity known to be present is not observed. In contrast, Eadie plots (velocity/substrate concentration versus velocity) are much more sensitive in revealing homotropic effects. Since the observed co-operativity depends on both the allosteric equilibrium constant, L, and the number of active sites, n, simulations were performed on the effect of those parameters. The maxima in the Eadie plots increased as L was lowered and conversely the maxima decreased as n was reduced. These changes were confirmed with a mutant aspartate transcarbamoylase which had the same specific activity as the wild-type enzyme and a lower value of L, and also with a hybrid enzyme containing fewer active sites and the same L value. Analogous experiments on nitrated aspartate transcarbamoylase derivatives of decreasing activity showed that Eadie plots were of value in distinguishing between the changes in L and n values resulting from the inactivation. Data from the literature were analyzed in the form of Eadie plots and in all cases homotropic effects were readily detectable for aspartate transcarbamoylase derivatives previously claimed to be devoid of co-operativity.     

Auxin-induced Conjugation Systems in Peas

Pretreatment of pea (Pisum sativum var. Alaska) sections with any active auxin induces an enzyme which forms aspartate conjugates of exogenously supplied indoleacetic acid, naphthaleneacetic acid, or benzoic acid. Whereas induction of this system is an absolutely auxin-specific process, another enzyme, which forms benzoylmalic acid, is induced both by auxins and by physiologically inactive aromatic carboxylic acids. Induction of both enzymes is abolished by low levels of RNA and protein synthesis inhibitors. The induction specificities and other characteristics of the two systems are compared.     

Trypanosoma rangeli: An alkaline ecto-phosphatase activity is involved with survival and growth of the parasite

The aim of this work was to investigate whether an alkaline ecto-phosphatase activity is present in the surface of Trypanosoma rangeli. Intact short epimastigote forms were assayed for ecto-phosphatase activity to study kinetics and modulators using β-glycerophosphate (β-GP) and p-nitrophenyl phosphate (pNPP) as substrates. Its role in parasite development and differentiation was also studied. Competition assays using different proportions of β-GP and pNPP evidenced the existence of independent and non-interacting alkaline and acid phosphatases. Hydrolysis of β-GP increased progressively with pH, whereas the opposite was evident using pNPP. The alkaline enzyme was inhibited by levamisole in a non-competitive fashion. The Ca²⁺ present in the reaction medium was enough for full activity. Pretreatment with PI-PLC decreased the alkaline but not the acid phosphatase evidence that the former is catalyzed by a GPI-anchored enzyme, with potential intracellular signaling ability. β-GP supported the growth and differentiation of T. rangeli to the same extent as high orthophosphate (P_i). Levamisole at the IC₅₀ spared significantly parasite growth when β-GP was the sole source of P_i and stopped it in the absence of β-GP, indicating that the alkaline enzyme can utilize phosphate monoesters present in serum. These results demonstrate the existence of an alkaline ecto-phosphatase in T. rangeli with selective requirements and sensitivity to inhibitors that participates in key metabolic processes in the parasite life cycle.     

Cerium Chloride Improves Protein and Carbohydrate Metabolism of Fifth-Instar Larvae of Bombyx mori Under Phoxim Toxicity

The organophosphorus pesticide poisoning of the silkworm Bombyx mori is one of the major events causing serious damage to sericulture. Added low-dose rare earths are demonstrated to increase resistance in animals. However, very little is known about whether or not added CeCl₃ can increase resistance of silkworm to phoxim poisoning. The present findings suggested that added CeCl₃ to mulberry leaves markedly increased contents of protein, glucose and pyruvate, and carbohydrate metabolism-related enzyme activities, including lactate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, and attenuated free amino acids, urea, uric acid and lactate levels and inhibited the protein metabolism-related enzymes activities, such as protease, alanine aminotransferase and aspartate aminotransferase in the haemolymph of B. mori, under phoxim toxicity. These findings suggest that added CeCl₃ may improve protein and carbohydrate metabolisms, thus leading to increases of growth and survival rate of B. mori under phoxim stress.